This research project is designed to determine the interrelationship between certain hormones (primarily gastrointestinal), intestinal blood flow and the transport of salt and water by the gut. Some gastrointestinal hormones alter gut absorption and also alter gut blood flow. It is known that changes in blood flow and/or blood pressure can alter salt and water transport across epithelial tissues. The question arises, are the effects of hormones due to a direct effect on transport by mucosal cells and/or an indirect effect mediated through the cardiovascular system. Total blood flow through a gut segment and also blood flow at the absorptive site will be measured as the clearance of 3H2O. Net and unidirectional Na and H2O fluxes will be measured by means of radioisotopes and flame photometry. Mesenteric arterial and venous pressure will be measured. Hormones will be infused intravenously and locally into the mesenteric artery and also released endogenously by perfusion of the gut with secretagogues. Changes in capillary pressure and flow at the absorptive site will be correlated with changes in unidirectional Na and H2O fluxes to determine if there may be a cause-effect relationship.